Increasing efforts have been put into research about thermoelectric materials for the last few decades, especially recently, faced with the crucial demand for new energy and energy savings. Among the potential candidates for new generation thermoelectric materials are the intermetallic clathrates. Clathrates are cage-structured materials with guest atoms enclosed. Previous studies have shown lower thermal conductivities compared with many other bulk compounds, and it is believed that guest atom vibration modes are the reason for such thermal behaviors. Several models, including the Einstein oscillator and soft potential models, have been used to explain the guest motion. However the characterization of the anharmonic oscillating motion can be a challenge. In this work, Nuclear Magnetic Resonance (NMR), heat capacity and transport measurements have been used to study several clathrate systems, especially the well- known type-I Ba8Ga16Sn30, which has been reported to have one of the lowest thermal conductivities for bulk compounds. In this material the strong anharmonic rattling behavior was investigated and analyzed according to a double well potential model, yielding good agreement with the experimental results. Furthermore, the resistivity and heat capacity results were studied and analyzed according to the influence of the anharmonic contribution. This offered a way to connect the NMR, transport and heat capacity properties, providing an advantageous way to study strongly anharmonic systems. In further work, several related intermetallic materials were examined for their structure, motion and NMR properties. Dynamical and electrical behaviors were investigated by studying the magnetic and quadrupole NMR spin-lattice relaxation. Type-VIII Ba8Ga16Sn30 exhibits an enhanced dynamics-related term at low temperature, but no rattling response as observed for the type-I structure. Type-I Ba8In16Ge30 was compared with the type-I Ba8Ga16Sn30 because their cage structures are similar. No strong anharmonic contribution was found in the NMR T1 behavior of Ba8In16Ge30, however the T2 showed behavior characteristic of atomic motion. In all cases, the magnetic relaxation was used to characterize the electron structures, and n- type Ba8Ga16Ge30 exhibited a spin-lattice relaxation behavior which is characteristic of impurity band structures near the Fermi surface. Also, a series of Ba8CuxGe46-x clathrates were investigated and showed much more insulating like behavior. In related work, the layered BaGa4 and BaGa3Sn have shown interesting NMR spin-spin relaxation behavior that indicates atomic fluctuations. This is similar to the situation found in type-I Ba8In16Ge30. The influence of atomic motion on the NMR and also the atomic structures of these alloys is further discussed in this work.

The present study aims to increase understanding of the physical processes that govern thermoelectric efficiency in Na-containing group 14 type II clathrates. This has been achieved through structural characterization and physical property measurements. Local and electronic structures of Si clathrates with the formula NaxSi136, where x = 0, 1.3, 5.5, 7.2, 8.8, 14.1, 20 and 21.5 were studied using x-ray absorption spectroscopy. Thermoelectric properties, namely Seebeck coefficient, electrical conductivity and thermal conductivity were measured from 2.5 K to 400 K. Low Na content samples, x < 8, showed reduced thermal conductivity compared to the empty clathrate, x = 0. For x > 8, increased Na content led to increased charge transfer, increased thermal conductivity and decreased magnitude of Seebeck voltages. The heat capacities of the NaxSi136 materials were measured from 2.5 K to 300 K. Analysis of the heat capacity data showed that the vibrational modes associated with Na in the Si28 cages are of sufficiently low energies to interact with heat transporting acoustic phonons, leading to reduced thermal conductivity as x is increased up to ~ 8. Increasing Na content beyond x = 8 introduces Na into the Si20 cages. This stiffens the lattice, increasing (or maintaining) phononic contributions to thermal conductivity, and increasing electronic contributions. Electronic thermal conductivity is responsible for upwards of 50 % of heat conduction when x = 21.5. Na containing type II Ge clathrates were produced using an ionic liquid reaction medium. Seebeck coefficients observed in Na9Ge136 materials, were negative but larger in magnitude than those of the NaxSi136 materials and thermal conductivities of Na9Ge136 were lower than those of the NaxSi136 materials. While both Si and Ge type II clathrates showed modest figures of merit, with maximum ZT values of 2.5 × 10-6 and 2.8 × 10-5 observed in Na20Si136 and Na¬9Ge136, of the two framework elements, type II Ge clathrates have been shown to have more favourable thermoelectric properties.